Magnetic and Ionospheric Observations During the October 24, 1995 Total Solar Eclipse in Vietnam

The track of the Moon's shadow during the Total Solar Eclipse of Oc­ tober 24, 1995 passed parallel to the magnetic Equator in the Vietnam sec­ tor. This fact together with its lengthy 2-minute duration at around noon time of totality provided an ideal opportunity to study the solar eclipse ef­ fect both on the global Sq and electrojet currents. In Vietnam, the recording of the magnetic variations in order to study the Equatorial Electrojet's effect has been carried out at 4 magnetic sta­ tions since 1987. These stations are situated in the region with the magnetic inclination (I) between 0°N and 34°5N. During the period of the Total Solar Eclipse of October 24, 1995, two new temporary magnetic stations were instaled and operated for one week at Phanthiet and Baoloc, two places directly lying under the belt of the Total Solar Eclipse. The eclipse effect on the H component was observed only in the region of the magnetic Equator (inside the range of 400 km from the magnetic Equator): the H component started to decrease immediately with the be­ ginning of the solar eclipse at 09.33 + 09.38 LT; at the moment of the maximum Sq(H) for October 1995 the .1 H with reference to the night time base value decreased to 76-82% of the value of the normal Sq. Ionospheric observations were carried out in Hanoi (with obscuration 0,78) by the Ionosondes IPS-71 made by KEL Aerospace Pty. Ltd. (Aus­ tralia). The analyses ofionograms showed that at the beginning of the local solar eclipse, foFl, with a slightly larger value than that of the monthly median , started to gradually decrease following the decrease in local solar to its minimum value around the time of local maximum occulta­ tion. After the end of the local eclipse, foFl returned to its monthly median value. These results suggest that the normal electric field in the Sq band was left unaffected and the eclipse effect on the geomagnetism in the region of the magnetic Equator the decrease in the ionization in the Equa-1


INTRODUCTION
The study of the effect of a solar eclipse on the geomagnetic field was first carried out as early as 1900 by Bauer. According to the theoretical discussion of Chapman ( 1933), the ideal conditions to clearly demonstrate the reduction in the geomagnetic Sq variation at the time of a solar eclipse are: a magnetically quiet day, the eclipse occuring around local noon when Sq attains its peak value, the observation site being located within the path of totality or closeby and a duration of several minutes for the totality. Chapman and Bartels (1940) pointed out that partial eclipses (with 70 per cent or more of the Sun obscured at its maximum phase) were as worthwhile as total eclipses for the observation of magnetic effects on the ground. Since the Sq variations are abnormally large in the Equatorial Electroject region as compared to those in low latitude stations situated outside the jet region, the eclipse reduction can also be expected to be quite large at the electrojet stations, even if the eclipse is not total.
The most intensive observations of the effects of an eclipse on the electrojet were done in Peru during the solar eclipse of November 12, 1966 (Giesecke et al., 1968). It was found that the maximum effect of the solar eclipse at different stations in Peru was proportional to the corresponding normal Sq variation. Rastogi ( 1982) reviewed the observations of solar eclipse effects at low latitudes and concluded that they are distinct on the electrojet when the tracks of totality are parallel to the magnetic Equator.
A Total Solar Eclipse occurred on October 24, 1995 over the Asian sector. It started during the sunrise hours in Iraq and terminated near Indonesia during pre-sunset hours. In the territory of Vietnam, the track of the Moon's shadow passed parallel to the magnetic Equator at the distance of 300km to the South (Figure 1). The important factor is that the path of the eclipse ran obliquely to the dip Equator, therefore providing opportunity to study the effect in both the global Sq and the electrojet regions. The facts that the time of totality was around noon (about 11. 20 LT) and the duration of totality was the longest in the Vietnam sector provided the ideal setting to qualify eclipse-induced changes in the magnetic field (Table 1).

RESULTS OF MA GNETIC OBSERVATIONS
In Vietnam, the recordings of magnetic variations so as to study Equatorial Electrojet's effect have been carried out at 4 magnetic stations since 1987. These stations are situated in the region with the magnetic inclination (I) between 0°N and 34°5N.
During one week both before and after the Total Solar Eclipse of October 24, 1995 the geomagnetic field was recorded at six stations in the territory of Vietnam : 2 new temporary stations were inside, while the other 4 regular stations were outside of the track of the Moon's shadow ( Figure 2). The positions of the geomagnetic stations in Vietnam during the Total Solar Eclipse on October 24, 1995 and the time of the eclipse on the ground are shown in Table 1. At all of the magnetic stations, the variometer type Bobrov (made at the Institute of Terrestrial Magnetism, Ionosphere and Radio Propagation, Moscow, Russia) with a paper speed of 20 mm/hour and sensitivity of H of about 2.5nT/mm were used to record magnetograms.  Figure 3. When compared with the magnetograms of every magnetic station of 24 October, 1995 with the mean of the quiet day variations in the H component for October 1995 from the same stations, it is interesting to note the following variations in the behavior in the region within the distance of 400 km from the magnetic Equator and those in the region father to the north of Vietnam at more than 1400 km from the magnetic Equator (see Figure 2).
In the first region (between the totality belt of the solar eclipse and the magnetic Equator) the maximum of the Sq(H) reaches 11.20 LT (see It is worth noting that the variations in both these parameters at the distance from the centre of the magnetic Equator are very similar to each other. Further more, the � H due to the eclipse has a linear relation with the � H due to the Sq current (Fig. 5).
The above results confirm the suggestion of Rastogi ( 1982) that the normal electric field in the Sq band remained unaffected, and the eclipse effect on the geomagnetic field in the  region of the magnetic Equator was due to the decrease in the ionization in the Equatorial Electrojet Current.

RESULTS OF IONOSPHERIC OBSERVATIONS
It is known from radio observations that during a solar eclipse the ionization of the E, F 1 and F2 layers is reduced (Chapman, 1962). In this study, the ionospheric observation was carried out at Hanoi ( <p= 21°02N, A= 105°58E dip=3 1°6N). The maximum percentage of obscuration (78%) of the solar disc occurrs at 10.57LT as seen from Hanoi on the eclipse day, October 24, 1995. The digital ionosonde system IPS-7 1 was operated continuously every 2 minutes during October 1995. The important ionospheric parameters foF2, foFl and h'Fl, h'F2 were scaled from the ionograms of the eclipse day (24th October 1995) and of the control days (23th and 25th October 1995).
The Eclipse of October 24, 1995 at Hanoi begins at 09. 28 LT, reaching a peak of 78% at 10.57 LT and ending at 12.32LT. It is found that because of strong absorption and sporadic E, the value of foE is not worth noting due to its lack of reliability.
The two-minute value of foFl and foF2 as a function of local time from 08.00 LT to 14.00LT on the eclipse day (October 24, 1995) and control days (October 23 and 25, 1995) are presented in Figure 6a, 6b. One can see that at the local solar eclipse, foFl starts to gradually decrease following the decrease in the local solar radiation to its minimum value around the time of maximum occultation and recovers gradually following the recovery of local solar radiation (Figure 6a). From Figure 6b, foF2 continues to increase after the start of the local eclipse as the case of two control days until 30 minutes before the maximum occultation at Hanoi, when foF2 reaches the stable value. Afterward, foF2 maintains a stable value until one hour before the end of the eclipse.
The variation of h'F2 and h'Fl as a function of local time from 08.00 to 14.00LT on the solar eclipse day October 24, 1995 and control day of October 23 and 25, 1995 are presented in Figure 7. No . reliable effect on h'Fl is observed, but in h'F2 the decrease of about 25 to 45 km in comparison with control days (23th October and 25th October) at the moment near the maximum occultation is noted.

CONCLUSIONS
1. In the Vietnam sector, the Total Solar Eclipse of October 24, 1995 provided an ideal oppor tunity for the observation of the eclipse effect on the geomagnetic field: a magnetically quiet day, with the eclipse occurring around local noon for a duration of about 2 minutes and the track of Moon's shadow passing parallel to the magnetic Equator.
2. The eclipse effect on the geomagnetic field was observed only in the region of magnetic Equator (within the distance of 400km from the magnetic Equator): the �H due to the eclipse had a linear relation with � H due to the Sq current.
3. At the local solar eclipse at Hanoi (obscuration 78% ), foFl changed in the same way as that of the local solar radiation.
4. These new results confirm the suggestion of Rastogi ( 1982) that the normal electric field in the Sq band remained unaffected, and the eclipse effect on geomagnetism in the region of the magnetic Equator was due to the decrease in the ionization in the Equatorial Electrojet Current.